The US State Department said on Thursday that credible reports of tank columns moving on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a serious escalation of the government's efforts to crush an armed rebellion.

"This is the concern: that we will see a massacre in Aleppo and that's what the regime appears to be lining up for," State Department spokeswoman Victoria Nuland said.

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Nuland said that despite US concern over the violence, the Obama administration did not contemplate ending its self-imposed ban on direct military help.

"We do not believe that pouring more fuel on the fire is going to save lives," Nuland said. "The route out of this is not more violence ... the route out of this is an end to the violence and a beginning to a true political transition process."

Al Jazeera's Zeina Khodr, reporting from the Turkish-Syrian border town of Masna'a, said that opposition activists say the clashes in Aleppo have already begun.

"There is fighting in a number of neighbourhoods [in Aleppo]. We do know that fighitng has been going on for the past few days, [with] the Free Syrian Army [an armed opposition group] managing to take control of some neighbourhoods. The government is fighting back," she reported.

Nuland, meanwhile, dismissed comparisons between Aleppo and the Libyan city of Benghazi, the cradle of the uprising against Muammar Gaddafi, where warnings of a threatened massacre by Libyan government forces in March 2011 prompted the UN Security Council to authorise military action to protect civilians.

"There are a vast number of differences," Nuland said, citing lack of a UN mandate, more difficult terrain and the absence of a direct call for help by a unified opposition.

"The vast majority of the Syrians want the violence to end. They don't want increased violence," Nuland said. "The kind of groundswell call for external support that we've seen elsewhere is not there."

According to Free Syrian Army spokesman Colonel Abdel Jabbar al-Okaidi, some 100 tanks and a large number of military vehicles had arrived outside Aleppo.

Troops stationed on the outskirts of Aleppo unleashed barrages of heavy-calibre mortar rounds on the western neighbourhoods of Saladin, al-Sukkari and al-Fardos, while Russian MI-25 helicopter gunships struck al-Sakhour in the east with rockets, several opposition activists in the city said.

Rebel fighters targeted army roadblocks and security installations, with both sides avoiding close-quarters warfare in the city of 2.5 million people, Syria's biggest urban centre.

Thirty-four people were killed in Aleppo and its environs on Thursday, according to opposition activists keeping a tally of casualties in the northern city.

On Friday, the Syrian Observatory for Human Rights (SOHR), a UK-based anti-government rights watchdog, reported that several towns in Deraa province had been bombarded, killing at least four people. It also reported that government troops were engaged in violent clashes with armed opposition forces in Idlib province.

Call for support

The heavy fighting around Aleppo and the capital Damascus mark a new phase of the violence that has gripped Syria since protests broke out some 16 months ago. It comes after a bomb attack that killed four of Assad's closest lieutenants in Damascus last week that led some analysts to speculate that the government's grip was failing.

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One of the most senior figures to defect from Assad's inner circle, Brigadier General Manaf Tlas, put himself forward on Thursday as someone who could help unify the opposition inside and outside Syria on a plan for a transfer of power.

Tlas, speaking in a newspaper interview in the Saudi city of Jeddah, also said he was looking for support from Saudi Arabia and other powers. "I am discussing with ... people outside Syria to reach a consensus with those inside," Tlas told Thursday's edition of the Saudi newspaper Asharq al-Awsat.

Tlas went on to Turkey and met with Foreign Minister Ahmet Davutoglu. Turkey, a former ally of Assad and now one of his fiercest critics, has a heavy strategic stake in shaping any post-Assad leadership in neighbouring Syria.

Tlas appeared briefly with Davutoglu at an official guest house, but made no statement.

World split

Turkey closed its border posts with Syria on Wednesday to all traffic except Syrian refugees, even as Ikhlas Badawi, a Syrian parliamentarian, "defected" to the country on Thursday.

Samir Nashhar, a member of the opposition Syrian National Council, told the AFP news agency that Badawi arrived in Turkey on Thursday, and will now head to Qatar, "which has agreed to receive her".

Russia, one of the few remaining allies of the authoritarian Assad whose family has run Syria for 42 years, said calls for him to quit power were hindering efforts to end the conflict.

Foreign Minister Sergei Lavrov said such calls, led by the United States, Turkey and other Western and Arab nations, were fanning violence. He reiterated Moscow's contention that support for Syrian rebel groups was tantamount to backing terrorism.

Iran promised to stand by Syria, come what may.

First Vice President Mohammad Reza Rahimi told Iran's Press TV on Thursday that the Islamic Republic's support for Syria, its main Arab ally, was "unchangeable", countering suggestions that Iran could soften its backing for Assad.